1. Field of the Invention
The present invention is directed to an amorphous, homogeneous and compatible solid polymer alloy electrolyte and a manufacturing method therefor, and to a composite electrode, a lithium polymer battery and a lithium ion polymer battery using the same and manufacturing methods therefor.
2. Description of the Background Art
A conventional solid polymer electrolyte for a lithium battery and the like has been mostly manufactured of polyethylene oxide (PEO), but recently a solid polymer electrolyte in a gel- or hybrid-type which has ion conductivity greater than 10xe2x88x923 S/cm at an ambient temperature is being developed. Especially, the solid polymer electrolytes which can be employed for a lithium polymer battery are a polyacrylonitrile-based (hereinafter, referred to as xe2x80x9cPANxe2x80x9d) solid polymer electrolyte in a gel-type which was described in U.S. Pat. No. 5,219,679 of K. M. Abraham et al. and U.S. Pat. No. 5,240,790 of D. L. Chua et al., and a polyvinylidene fluoride-based (hereinafter, referred to as xe2x80x9cPVdFxe2x80x9d) solid polymer electrolyte in a hybrid-type which was described in U.S. Pat. Nos. 5,296,318 and 5,460,904 of A. S. Gozdz et al., the disclosures of which are incorporated hereinto by reference.
The gel-type PAN-based electrolyte has excellent adhesion, so that a composite electrode can be firmly adhered to a metal substrate. There are thus advantages in that a contact resistance is small in charging or discharging a battery and an active material is less separated. However, the electrolyte is more or less soft, thereby reducing mechanical stability, namely strength. Such weak strength may cause a serious problem in manufacturing the electrode and battery.
The hybrid-type PVdF-based electrolyte is manufactured by making a polymer matrix have a porosity of less than a submicron and by injecting an organic solvent electrolyte into the pore. It has excellent compatibility with the organic solvent electrolyte, and can be stably used because the organic solvent electrolyte injected into the small pores does not leak. Besides the polymer matrix can be manufactured in the ambient atmosphere because the organic solvent electrolyte is later injected. However, a manufacturing process therefor is quite complicated, that is a process for extracting a plasticizer and a process for infiltrating the organic solvent electrolyte are required in manufacturing the solid polymer matrix. In addition, while the mechanical strength of the PVdF-based electrolyte is quite excellent, the adhesion thereof is inferior, thereby requiring a process for making a thin layer by heating and the extracting process in manufacturing the electrode and battery.
According to an article in Solid State Ionics, 66, 97, 105(1993) by O. Bohnke, G. Frand et al., a poly(methyl methacrylate)-based (hereinafter, referred to as xe2x80x9cPMMAxe2x80x9d) solid polymer electrolyte has an ion conductivity of approximately 10xe2x88x923 S/cm at an ambient temperature, strong adhesion and excellent compatibility with the organic solvent electrolyte. However, the mechanical strength thereof is extremely weak, and thus not appropriate for a lithium polymer battery. In addition according to an article in J. Electrochem. Soc., 140, L96(1993) by M. Alamgir and K. M. Abraham, a poly(vinylchloride)-based (hereinafter, referred to as xe2x80x9cPVCxe2x80x9d) solid polymer electrolyte has an ion conductivity of approximately 10xe2x88x923 S/cm at an ambient temperature and excellent mechanical strength. However, there are disadvantages in that the low temperature characteristics are inferior and the contact resistance is great.
Also a solid polymer electrolyte manufactured by a blending was develpoed. For instance, U.S. Pat. No. 5,585,039 of M. Matsumoto et al. which was filed by NTT in Japan is directed to a solid polymer electrolyte including a multi-phase polymer matrix and an organic solvent electrolyte solution. The poly-phase polymer matrix includes a highly polar polymeric (HPP) phase and a less polar polymeric (LPP) phase. The HPP phase exhibits ion conductivity by infiltrating the organic solvent electrolyte, and the LPP phase is used as a supporting body. Accordingly, the polymers which are generally used for manufacturing the solid polymer electrolyte, such as PEO, PAN, PVdF, polypropylene oxide and the like are employed for the HPP phase. The polymers which are used as a supporting body, such as polystyrene, polypropylene, polyethylene and the like can be utilized for the LPP phase. Such a solid polymer electrolyte has an ion conductivity between 10xe2x88x924 and 10xe2x88x923 S/cm at an ambient temperature which is not sufficient for a lithium polymer battery. Especially, the ion conductivity at a temperature below 0xc2x0 C. is inferior. This solid polymer electrolyte is unsatisfactory for the lithium polymer battery because it does not have the essential characteristics which the lithium polymer battery should have, such as adhesion to the electrode, compatibility with the organic solvent and mechanical strength. Besides, this solid polymer electrolyte is poly-phased, and the polymers are in separated phases, and thus the phase separation proceeds by the repetition of the charge/discharge cycle and temperature cycle. As a result, the battery performance is rapidly deteriorated.
On the other hand, U.S. Pat. Nos. 5,631,103, 5,639,573 and 5,681,357 of M. Oliver et al. of Motorola in U.S.A. are basically identical in concept to the above-described invention of NTT in Japan, namely directed to a method for manufacturing a blended solid polymer electrolyte. The solid polymer electrolyte according to the above-mentioned patents includes two different phases. A first phase serves to absorb the organic solvent electrolyte and a second phase does not absorb the organic solvent electrolyte Without reactivity, and serves to prevent a gel electrolyte from being expanded, to be used as a supporting body and to increase the mechanical hardness. The first phase generally includes PVdF, polyurethane, PEO, PAN and the like which are used for manufacturing the solid polymer electrolyte, and the second phase includes polyethylene, polypropylene, polytetrafluoro-ethylene (PTFE) and the like. This solid polymer electrolyte has an ion conductivity of approximately 10xe2x88x924 S/cm at an ambient temperature, and thus is more or less weak for an ambient temperature type lithium polymer battery. Especially, the ion conductivity at a temperature below 0xc2x0 C. is inferior. Besides, the solid polymer electrolyte does not have the essential properties for the lithium polymer battery, such as adhesion to the electrode, compatibility with the organic solvent electrolyte and mechanical strength, similarly to the solid polymer electrolyte of NTT in Japan, and thus is not suitable for a lithium polymer electrolyte. In addition, the solid polymer electrolyte includes a heterogeneous phase, and thus the phase separation is increased by the repetition of the charge/discharge cycle and temperature cycle, which rapidly reduces the battery performance.
On the other hand, a conventional normal temperature type lithium secondary battery has been firstly developed by Sony in Japan. Here, a lithium ion battery has been worldwidely used and a lithium polymer battery is expected to be widely used in a few years.
The lithium ion battery is a separator, namely a PE (polyethylene) or PP (polypropylene) separator is used. It is difficult to manufacture a battery by stacking the electrode and separator in a flat-plate form. Therefore, the battery is manufactured by rolling up the electrode and separator and inserting them into a cylindrical or rectangular tube (D. Linden, Handbook of Batteries, McGRAW-HILL INC., New York(1995)). Although the lithium ionic battery has been widely used, there are still some problems for safety. Besides, the process for manufacturing the battery is complicated, which results in low productivity. Further, the battery shape is limitedly selected and the battery capacity is restricted. The lithium polymer battery is expected to overcome the above-described disadvantages. The lithium polymer battery employs a solid polymer electrolyte which functions as both the separator and electrolyte. The battery can be manufactured by stacking the electrode and polymer electrolyte in a flat-plate form. In addition, the manufacturing method therefor is similar to the method for manufacturing the polymer film, which results in enhanced productivity. However, it is not yet widely used because a solid polymer electrolyte has not been developed which has adhesion to the electrode, mechanical strength, low and high temperature characteristics and compatibility with the organic solvent electrolyte for the lithium secondary battery.
Recently, there has been developed the hybrid-type PVdF-based solid polymer electrolyte which was described in U.S. Pat. Nos. 5,296,318 and 5,460,904 of A. S. Gozdz et al. There is thus a plan to mass-produce the hybrid-type lithium polymer battery sooner or later. However, according to this battery system, a plasticizer is used in manufacturing the solid polymer electrolyte and cathode/anode, and thus a process for extracting the plasticizer and a process for infiltrating the organic solvent electrolyte are further required, which results in a complicated manufacturing process. Besides the PVdF-based electrolyte has a strong mechanical strength, but inferior adhesion. Accordingly, a process for making a thin film by heating needs to be carried out in manufacturing the electrode and battery. The battery performance is reduced because the electrode and solid polymer electrolyte are separated during the extracting process. In addition there is a disadvantage in that the porosity in the electrode is relatively high, as compared with the lithium ion battery, and thus the organic solvent electrolyte is much infiltrated, thereby decreasing energy density and deteriorating high-rate charge/discharge characteristics. Furthermore, the hybrid-type PVdF-based electrolyte has inferior low and high temperature characteristics.
It is therefore a primary object of the present invention to provide a solid polymer alloy electrolyte in a homogeneous state which has superior properties, such as ion conductivity, adhesion to the electrode, compatibility with an organic solvent electrolyte, mechanical strength and the like, and a manufacturing method therefor.
It is another object of the present invention to provide a composite cathode and a composite anode using the solid polymer electrolyte manufactured according to the present invention, and a manufacturing method therefor.
It is still another object of the present invention to provide a high performance lithium polymer battery which has excellent energy density, cycle life characteristics, low and high temperature characteristics, and high-rate discharge characteristics, by using the solid polymer alloy electrolyte and composite cathode(anode) according to the present invention, and a manufacturing method therefor.
It is still another object of the present invention to provide a lithium ion polymer battery which makes the best use of the above-described advantages of the lithium ion battery and lithium polymer battery and has superior energy density, cycle life, low and high temperature characteristics, high-rate discharge characteristics and stability, and a manufacturing method therefor. The method for manufacturing a lithium ion polymer battery according to this invention is relatively simple and an enlargement of the battery is possible.